This invention relates to control apparatus for providing uniform advancement of a flexible strip, such as a flexible information-bearing medium, which is concurrently advanced at another location with intermittent motion. An example of such dual motions of a strip occurs in a sound motion picture projector, where a portion of the film is advanced step-wise past a visual projection station, while another portion is advanced uniformly past an audio station. To accommodate the two motions, a buffer loop is introduced between the two stations. Without synchronization, however, the buffer loop would contract and disappear, or grow excessively. Thus, where the drive mechanisms of intermittent and of uniform motions are mechanically independent, a system is needed for controlling the speed of the two mechanisms in relation to each other.
Theoretically, either the film drive mechanism or the audio drive mechanism, or both, can be regulated or controlled. However, excessive fluctuation in the speed of the audio medium past the sound transducer results in audio distortion. Thus, where design constraint rules out regulation or control of the film drive mechanism, the rate of control of the audio drive must be carefully limited. One approach to this problem is illustrated in U.S. Pat. Nos. 3,856,387 issued Dec. 24, 1974 to Wray et al. and 3,832,044 issued Aug. 27, 1974 to Deeran, both assigned to the assignee of the present application and both incorporated by reference herein. These patents disclose systems in which an alternating current (AC) film drive motor is unregulated, and a direct current (DC) audio capstan drive motor is controlled in response to the buffer loop level by a servomechanism having a relatively long time constant. A servomechanism or control system with a sufficiently long time constant to avoid audible distortion, however, is apt to be unable to seize initial control of the loop at the selected height if the loop starts growing more rapidly than the rate at which it will change dimension under normal steady-state operation. This, of course, limits the speed at which the loop can be formed initially. At the same time, however, it is desirable to improve even further upon the fidelity of audio reproduction by eliminating possibly perceptible speed changes in the audio capstan while maintaining a selected loop size.
In the initial establishment of the selected loop size, a further problem is encountered if the control system overreacts to detection of the loop and produces successively diminishing overcorrections in opposite directions. In this condition the speed at which the medium is uniformly advanced oscillates in an "under-damped" fashion above and below the nominal speed. Overcorrection will occur even though the medium continues to be intermittently advanced at a relatively constant speed. It would be desirable to arrive at the nominal speed of uniform motion more quickly and more directly once the loop is detected.
Although sound tracks are typically placed alongside the motion picture frames on the same strip, another system uses audio magnetic recording tape interwound with a separate film strip. This system, which has many advantages, especially for film chemically processed in a cassette, is described in detail in copending U.S. patent applications Ser. No. 869,131 filed Jan. 13, 1978 by Land et al. and Ser. No. 899,864 filed Apr. 25, 1978 by Wray, and both assigned to the assignee of this application and incorporated herein by reference. However, the two-strip system still requires synchronization of the uniform audio drive mechanism with the intermittent film drive mechanism.
The term "tape" is used herein to embrace information-bearing flexible media movable relative to an information-transferring transducer element, and whether encoded magnetically, or optically, and whether recorded in analog or digital fashion.
Accordingly, a general purpose of this invention is to provide an improved control apparatus and a method for synchronizing the concurrent intermittent and uniform advancements of a flexible strip.
A more particular object of the invention is to enable a servomechanism to seize initial control quickly of the height of a recording tape buffer loop without unduly interfering with audio reproduction from the tape.
Another object is to meet the above goals with a system which is applicable to film strips with sound tracks as well as to separate film and tape systems. Related objects include achieving the above objectives with reliable circuitry of minimum complexity.